The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This invention relates to methods for servicing subterranean wells, in particular, fluid compositions and methods for operations during which the fluid compositions are pumped into a wellbore, make contact with subterranean formations that comprise carbonate rocks, react with the rock surfaces and form a gel that seals voids and cracks.
During construction of a subterranean well, drilling and cementing operations are performed that involve circulating fluids in and out of the well. The fluids exert hydrostatic and pumping pressure against the subterranean rock formations, and may induce a condition known as lost circulation. Lost circulation is the total or partial loss of drilling fluids or cement slurries into highly permeable zones, cavernous formations and fractures or voids. Such openings may be naturally occurring or induced by pressure exerted during pumping operations. Lost circulation should not be confused with fluid loss, which is a filtration process wherein the liquid phase of a drilling fluid or cement slurry escapes into the formation, leaving the solid components behind.
Lost circulation can be an expensive and time consuming problem. During drilling, this loss may vary from a gradual lowering of the mud level in the pits to a complete loss of returns. Lost circulation may also pose a safety hazard, leading to well-control problems and environmental incidents. During cementing, lost circulation may severely compromise the quality of the cement job, reducing annular coverage, leaving casing exposed to corrosive downhole fluids, and failing to provide adequate zonal isolation. Lost circulation may also be a problem encountered during well-completion and workover operations, potentially causing formation damage, lost reserves and even loss of the well.
Lost-circulation solutions may be classified into three principal categories: bridging agents, surface-mixed systems and downhole-mixed systems. Bridging agents, also known as lost-circulation materials (LCMs), are solids of various sizes and shapes (e.g., granular, lamellar, fibrous and mixtures thereof). They are generally chosen according to the size of the voids or cracks in the subterranean formation (if known) and, as fluid escapes into the formation, congregate and form a barrier that minimizes or stops further flow.
Surface-mixed systems are generally fluids composed of a hydraulic cement slurry or a polymer solution that enters voids in the subterranean formation, sets or thickens, thus forming a seal that minimizes or stops further flow. Downhole-mixed systems generally consist of two or more fluids that, upon making contact in the wellbore or the lost-circulation zone, form a viscous plug or a precipitate that seals the zone. A thorough overview of LCMs, surface-mixed systems and downhole-mixed systems is presented in the following reference: Daccord G, Craster B, Ladva H, Jones T G J and Manescu G: “Cement-Formation Interactions,” in Nelson E B and Guillot D (eds.): Well Cementing—2nd Edition, Houston: Schlumberger (2006): 202-219.
Downhole-mixed systems may suffer from various drawbacks, largely arising from limited direct control of the manner by which the two fluids commingle in the well. Operators may encounter difficulties sealing a particular zone or treating long intervals.
U.S. Pat. No. 6,189,615 relates to the reduction of permeability or fluid mobility within a carbonate-containing treatment region in a well bore. The process is initiated by preparing a gelation solution at the earthen surface which includes an acrylamide polymer, a crosslinking agent (chromium III), a stabilizing agent and an aqueous solvent. The gelation solution is injected into the well bore and displaced into the treatment region where it is gelled in situ to form a gel which substantially reduces the permeability of or fluid mobility within the treatment region. The presence of a cross-linker in the solution may cause premature gelation.
U.S. Pat. No. 4,630,678 discloses a water permeability contrast correction process to improve the sweep efficiency of waterflooding in carbonate-bearing strata which involves a sequential injection of (1) an optional aqueous preflush slug to adjust connate water salinity, (2) a slug of aqueous polymer gelable with polyvalent metal cations, such as an polyacrylamide, (3) an acidic aqueous solution effective to dissolve some of the carbonate and at least therefrom polyvalent metal cations specifically calcium or magnesium or both effective to gel said polymer, wherein said aqueous polymer and acidic solution can be injected together in a single slug, followed by (4) an additional aqueous polymer solution injection, wherein the acidic solution provides polyvalent metal cations in-situ for gelation of said polymer slug to preferentially decrease water permeability in highly permeable thief zones, and (5) an aqueous drive fluid.
US Patent Application 2006/0122071 teaches a two-fluid system comprising an alkali swellable latex and a pH-increasing material. When the latex fluid commingles with the other fluid containing the pH-increasing material, raising the pH to levels between about 7 to about 14, a gel forms that acts as a seal.
WO 2009/074658 is similar to U.S. 2006/0122071 except that it involves pH lowering additives in order to delay the swelling reaction. The lowered pH is from about 4 to about 7. Basically, when the pH downhole increases to alkaline values, the alkali swellable latex forms a gel acting as a seal.
As apparent, there is still a need for a composition that will solve the problem at pH lower than 7, and ideally the solution would be “a one-fluid system” that does not rely upon fluid mixing to achieve sealing and that do not need the presence of a cross-linking agent to avoid premature reaction.